This invention relates generally to the treatment of non-toxic wastewater, including water-borne waste material from residential, commercial and other sources, and particularly to an improved system for nitrogen reduction.
Wastewater treated by conventional wastewater treatment systems contains soluble, partially soluble and insoluble material as well as contaminates. Materials in the wastewater may be decomposable, partially decomposable or not decomposable. Decomposable and partially decomposable materials are referred to as biodegradable; that is, the material may be biologically broken down, or stabilized by bacterial action. Wastewater treatment systems are designed to provide controlled decomposition of wastes to reduce pollution, health hazards and offensive odors.
Decomposable material is stabilized in wastewater treatment systems by bacteria, protozoa, and other microorganisms. Bacterial consumption of material, creating energy and reproducing bacterial cells, is the foundation of activated sludge wastewater treatment. Conventional wastewater treatment systems may include pretreatment, primary treatment, secondary treatment, and advanced treatment.
Pretreatment includes screening, comminuting (mechanical cleaning of screens by shredding solids to a size which can pass through screen openings), degritting, and grease and scum removal.
Primary treatment includes removal of suspended solids from wastewater by clarification and skimming. This typically involves a tank or channel and the following steps: reducing flow velocity, settling heavier solids, and skimming relatively light solids. Primary treatment may include anaerobic digestion processes, aerobic digestion processes, or a combination thereof. Primary treatment systems typically include sludge collection mechanisms, sludge suction devices, grit removal devices, and sludge dewatering devices to reduce the volume of sludge to be disposed.
Secondary treatment systems are typically aerobic systems including an aeration phase and a clarification phase. Secondary treatment systems typically include an aeration tank, an air distribution system, a clarifier, sludge collection mechanisms, and sludge removing devices.
Advanced treatment includes further removal of suspended and dissolved organic solids by means including filtration and removal of pathogens and chloroforms by oxidation, chlorination or heating, precipitation of minerals, adsorption, or other methods.
In the activated sludge process of primary or secondary treatment, microorganisms are contained in an activated sludge and mixed with incoming wastewater; the wastewater providing food for the microorganisms. Such mixing is accomplished in an aeration tank or channel. In the aerobic activated sludge process, oxygen is intimately mixed with the activated sludge and the wastewater. The microorganisms convert suspended organic solids into energy, carbon dioxide, water, and additional microorganism cells. The aerobic activated sludge process therefore typically includes mixing of wastewater, activated sludge, and oxygen in an aeration tank; consumption of suspended organic solids by bacteria; settling of activated sludge in the clarifier; returning the activated sludge to the aeration tank for further treatment; removing purified liquor from the clarifier; and removing and disposing of the final, inert sludge.
In the further process of advanced treatment, the purified liquor from the clarifier is typically filtered. The filtered liquor is refined through chlorination, oxidation, or heating.
This invention relates to a system for nitrogen reduction in wastewater. Nitrogen is a critical element required for protein synthesis and is essential to life. When living things die or excrete waste products, nitrogen that was tied to complex organic molecules is converted to ammonia by bacteria and fungi. In this state, the ammonia exerts a significant and undesirable oxygen demand on the environment as it enters lakes, streams and other bodies of water.
In conventional anaerobic septic systems, the ammonia present in the influent, due to the lack of oxygen necessary for chemical conversion of the ammonia to harmless forms of nitrogen and oxygen, is passed through the system to the drain or discharge field with the potential to reach a body of water untreated.
Wastewater systems utilizing an aerobic process undergo nitrification, or the conversion of ammonia to nitrites and the further conversion of nitrites and nitrates. This process is accomplished by two bacteria generaxe2x80x94Nitrosomonas and Nitrobacter. Nitrosomonas oxidizes ammonia to produce nitrite. Nitrobacter converts some of the produced nitrite to nitrate. These equations for the reactions that occur can be written as follows:
Nitrosomonas equation:
55NH4++76O2+109HCO3xe2x88x92xe2x86x92C5H7O2N+54NO2xe2x88x92+57H2O+104H2CO3
Nitrobacter equation:
400NO2xe2x88x92+NH4++4H2CO3+HCO3xe2x88x92+195O2xe2x86x92C5H7O2N+3H2O+400NO3xe2x88x92
In either the ammonia (NH4+) or nitrate (NO3xe2x88x92) form, damage to the environment or human health can result. Nitrogen is one of the nutrients required for growth. Excessive amounts can result in algae blooms and other problems. This is especially important where effluent is discharged to lakes and streams. The result is called eutrophication.
As discussed, conventional systems leave the nitrogen in an unacceptable state. It is important to convert the ammonia and nitrate to a form that can be released to the environment without causing harm. One treatment process is nitrification/denitrification of the wastewater by biological processes and subsequent release of gaseous nitrous oxide and molecular nitrogen into the atmosphere.
Once the ammonia has been converted to nitrates through nitrification, denitrification can be introduced to convert the nitrogen in the nitrates to an acceptable form. Facultative bacteria under anoxic conditions carry out denitrification. It is important that the oxygen level be reduced. Dissolved oxygen is an inhibitor to denitrification reactions. The reactions for nitrate reduction can be written as follows:
NO3xe2x88x92xe2x86x92NO2xe2x86x92NOxe2x88x92xe2x86x92N2Oxe2x86x92N2 with oxygen being released to the wastewater.
Bacteria required for the conversion of nitrogen compounds to nitrogen are sensitive organisms and extremely susceptible to a wide variety of inhibitors. A variety of organic and inorganic agents can inhibit the growth and action of these organisms, such as high concentrations of ammonia or nitrous acid. The effect of pH is also significant. A narrow range of between about pH 7.5 to about pH 8.6 is optimal. Temperature, either too high or too low, also has a significant effect on the growth of the bacteria and their ability to convert nitrogen compounds to nitrogen. A temperature range of about 60xc2x0 F. to about 100xc2x0 F. is ideal. Dissolved oxygen must be present for nitrification to occur and absent for the denitrification process. In both phases organic materials must be available to provide energy to the microorganisms for nitrogen compound conversion and for cell growth.
Conventional biological systems generally use multiple stages in removed tanks to achieve conversion of nitrogen to an acceptable form. The first stage is a pretreatment system for removal of solids and pretreatment of the wastewater. The second stage is an aerobic process for nitrification. The third stage is a separate biological system using methanol as the carbon source for denitrification. The Bardenpho Process, for example, utilizes the carbon from the untreated wastewater and from endogenous decay by returning the aerobically treated wastewater to the initial anaerobic zone. The partially treated effluent is then passed through another anoxic denitrification zone and a final aerobic zone and then through a secondary clarifier. The steps are performed in separate vessels.
In an xe2x80x9coxidation ditch process,xe2x80x9d mixed liquor flows around a loop-type channel. An aerobic zone is established immediately downstream of an aerator and an anoxic zone is created upstream of the aerator. The influent wastewater is injected at the upstream limit of the anoxic zone. This allows some of the wastewater carbon to be used for denitrification. The effluent of the system is taken at the end of the aerobic zone and transported to a clarifier. A method for the removal of the nitrous oxide and nitrogen gas may be added at this stage. This process is completed in a single circular containment vessel, on a scale that is feasible only for very large applications, such as for municipalities or communities.
Most biological nitrification/denitrification systems employ some modification of the described systems. They may incorporate suspended-growth, attached-growth, complete-mix, and plug-flow reactors in the process. Each process, in general, relies on anaerobic-aerobic-anaerobic flow to achieve nitrogen removal.
Current technology is generally practiced in relatively large plants providing wastewater treatment for communities. Many plants provide a high amount of process control. Conventional design of such plants, however, requires the use of a large number of mechanical subsystems including pumps, blowers, gears, chains, and associated mechanical elements. The large quantity of mechanical parts makes such conventional systems expensive to construct and maintain, as well as difficult to operate and infeasible for small applications.
U.S. Pat. No. 6,103,109 to Noyes et al., which is fully incorporated herein by reference, solves many of the problems of conventional designs. However, the vertical arrangement of the fixed media zone above the suspended micromedia compartment allows cellulose from dead microorganisms to fall into, and therefore clog, the pores of the suspended micromedia compartment. Therefore, a need exists for a wastewater treatment system in which the different treatment compartments are arranged so that clogging of the suspended micromedia compartment is minimized.
The present invention is a wastewater treatment system comprising a simple, compact, and economical design in a single tank with an improved flow schematic. It provides a mechanism for the reduction of nitrogen in effluent wastewater by combining several processes into a simple system operated by one air compressor. A key component of the invention is the combining of several process stages into one unit or tank. The result is a smaller, more efficient system able to meet stringent discharge requirements.
A preferred embodiment of the tank of the present invention comprises three compartments: an aerobic treatment compartment for nitrification, an anoxic treatment compartment for denitrification, and a suspended micromedia compartment. It also includes either a fourth compartment which serves as a discharge compartment, or a discharge well. The compartments are laterally adjacent to each other within the tank, so that solid particles that settle out of the wastewater in one compartment do not flow into, and thereby cause clogging in, another compartment.
The compressor or air pump used to provide oxygen for the aerobic process also provides the pumping action required to circulate the wastewater through the system and provide for nitrogen stripping in the final phase of treatment. The same compressor also provides the negative pressure required to facilitate the reduction of dissolved oxygen levels in the anoxic zone.